Example:The product claiming "all natural ingredients," which appeared in the Sunday newspaper, is on sale.The product is already identified. Therefore, which begins a nonessential clause containing additional, but not essential, information.

NOTE

Essential clauses do not have commas introducing or surrounding them, whereas nonessential clauses are introduced or surrounded by commas.

Rule 3. If that has already appeared in a sentence, writers sometimes use which to introduce the next clause, whether it is essential or nonessential. This is done to avoid awkward formations.

Example:That which doesn't kill you makes you stronger.
This sentence is far preferable to the ungainly but technically correct That that doesn't kill you makes you stronger.

NOTE

The distinction between that and which, though a useful guideline, is widely disregarded: Which is routinely used in place of that, even by great writers and journalists, perhaps because it sounds more elegant.

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